1. Field of the Invention
This invention relates in general to bottom cycle engines for utilizing the waste exhaust heat of an engine to produce mechanical work. In particular it relates to an improved reciprocating hot air engine that provides a simple, low cost to manufacture, means for recovering energy otherwise lost in the exhaust of an internal combustion, gas turbine or similar heat engine.
2. Description of Prior Art
The rising cost of fuels and concern about the environmental effects of burning those fuels have increased the need for developing novel methods for obtaining the maximum amount of heat and mechanical power from the combustion process. One very effective measure is to recover as much of the heat as possible from the exhaust gases. There are two general types of combustion processes, low pressure and high pressure, and each type has optimal methods for recovering the exhaust heat.
Low pressure combustion processes are used by steam boilers and industrial heating processes. In both cases, combustion occurs in a furnace that is essentially at atmospheric pressure. The most direct method for reducing the fuel requirements in these low-pressure processes is to use a counterflow heat exchanger to heat the incoming air with the hot exhaust gases. Alternatively, mechanical work can be obtained from a bottom cycle engine that is heated by the exhaust. The most effective method of recovering heat from a low-pressure combustion process is to combine both the air preheating and mechanical work recovery methods with an Afterburning Ericsson Cycle Engine as described in my U.S. Pat. No. 5,894,729 (1999). This engine can integrate the furnace process into the engine process by simply using the furnace as the afterburner. With this engine, the furnace receives a forced draft of hot air from the engine expander exhaust and the furnace then provides the hot exhaust back to the engine to produce mechanical work.
High-pressure combustion processes are the most common type of engine combustion process and are found in spark-ignition, Diesel, and gas turbine engines throughout the world. Their wide use makes them an ideal market for devices to effectively recover exhaust heat. Because their combustion process is high pressure, it cannot be integrated as the low-pressure afterburner of an Afterburning Ericsson Cycle engine. Instead, gas turbines can use a heat exchanger (generally termed a recuperator) to use the hot exhaust from the expander turbine to preheat the air from the compressor turbine to greatly improve the gas turbine engine efficiency. Nevertheless, adding a recuperator greatly alters the engine because of the need to insert a large heat exchanger into the otherwise compact engine. For this reason it is very difficult, and often impossible, to modify an existing, aircraft type, gas turbine engine for a recuperator.
Spark-ignition and Diesel engines are not able to use a recuperator and instead frequently use some form of xe2x80x9cbottom cyclexe2x80x9d where another engine is attached to the exhaust to use the exhaust heat and/or pressure to drive another engine. Such a system of two engines is called a combined cycle engine and consists of the spark-ignition, Diesel, or gas turbine engine (the top cycle engine) and a bottom cycle engine that is attached to the top cycle engine""s exhaust.
Gas turbine top engines are commonly joined with Rankine cycle bottom engines to make a very effective combined cycle engine that is widely used in large powerplants. The Rankine cycle makes a very effective bottom cycle that can effectively use much of the top cycle exhaust heat. However, the complexity and potential safety issues of the Rankine cycle engine are not justified for smaller, xe2x80x9cmicro-generationxe2x80x9d applications of less than 100-kilowatt output that are now entering the distributed power market.
Turbochargers have become the most common form of bottom cycle engine for spark-ignition and Diesel top cycles. Turbochargers use the hot, high pressure, exhaust from the top cycle to spin a turbine that is connected to a compressor that boosts the pressure of the air entering the top cycle engine. The boost pressure increases the mean effective pressure of the top cycle engine and increases its power. Turbochargers work very well with Diesel engines because they have no combustion limits from the increased boost pressure. There is a penalty in reliability and durability however.
Although a turbocharger can increase the power of a spark-ignition engine, there is usually very little gain in efficiency. The increased boost pressure leads to increased risk of detonation. Consequently, turbocharged engines are xe2x80x9cdetunedxe2x80x9d from their normally aspirated versions by reducing the compression ratio and adjusting the ignition timing. As a result, although capable of increased power, a turbocharged spark-ignition engine frequently xe2x80x9c . . . lowers fuel economy in comparison to the same engine naturally aspirated. The decision to use supercharging in this way is more one of marketing than one of utility.xe2x80x9d [Taylor, Charles Fayette: xe2x80x9cThe Internal Combustion Engine in Theory and Practice, Vol IIxe2x80x9d, The M.I.T. Press (1995) p. 367].
It would seem that a bottom cycle based on Stirling or Ericsson Cycle engines would be ideal. The theoretical efficiency of both these engines is the same as a Carnot enginexe2x80x94the maximum efficiency possible with a supply of heat at one temperature and a reservoir at a lower temperature for receiving the exhaust. However, a bottom cycle engine meets only half the Carnot engine requirements; although the surroundings provide the necessary constant temperature reservoir, the heat from a top cycle exhaust is not available at a constant temperature.
FIG. 1 shows a temperature-entropy diagram of a typical top cycle and resulting exhaust heat loss. Top cycle engines operate by taking in air at state 1, compressing it to state 2, then heating in either an approximately constant pressure or constant volume process from state 2 to state 3, and finally expanding it from state 3 to the exhaust at state 4. Because FIG. 1. is a temperature entropy diagram, the potential for generating mechanical work from the heat wasted in the top cycle exhaust is defined by the shaded area, A-1-4-A. A-1-4-A describes the difference between the heat available to the bottom cycle from the top cycle exhaust and the potential heat rejection to the surroundings by the bottom cycle. A bottom cycle engine must then be one that is capable of operating at maximum efficiency with a supply of heat of heat obtained by cooling the top cycle exhaust from state 4 to state 1 while rejecting its own heat at nearly the temperature of the surrounding environment, temperature 1-A. The ideal engine is one that best fills that area; A-1-4-A.
FIG. 2 shows an attempt to use an ideal Stirling engine cycle as a bottom engine. The Stirling engine is a closed cycle engine that starts with a low pressure gas at state A, the temperature of the environment. The gas is compressed at constant temperature to a higher pressure at state B, heated in a regenerator from state B to state C, and expanded in an expander from state C to state D. It is then cooled back to the surrounding temperature, state D to state A, in the regenerator (by giving up the same heat used to warm it from state B to state C). The Stirling engine receives heat from the top cycle exhaust during the expansion process, state C to state D; and rejects it to the environment in the compression process of state A to state B.
Although it efficiently uses what heat it can extract from the top cycle, the Stirling engine is not efficient in obtaining that heat. First, the upper temperature of the cycle (temperature of states C and D) is limited by a heat balance across the expander heat exchanger. The enthalpy change in the top cycle exhaust in going from its state 4 to the temperature of state C, (H4xe2x88x92HC)top, is equal to the Stirling engine""s upper temperature multiplied by the entropy change between state D and state C:
[Wdot Tc-d (Sdxe2x88x92Sc)]Stirling=[Wdot (H4xe2x88x92HC)]topxe2x80x83xe2x80x83(1)
where Wdot is the respective mass flowrate.
Because Tc-d is less than the potential peak temperature, T4, the potential to make work is reduced. That reduction is indicated in the xe2x80x9cPeak Temperature Heat Lossxe2x80x9d area in FIG. 2.
Second, the Stirling engine cannot cool the top cycle exhaust any lower than the temperature at state C and D, Tc-d. The exhaust from the top cycle will still be at that high temperature. The inability to extract heat from the top cycle represents a major loss in the potential to make work and is shown by the xe2x80x9cTop Cycle Exhaust Heat Lossxe2x80x9d in FIG. 2.
An Ericsson cycle engine has essentially the same limitations as the Stirling in adapting to a bottom cycle. Referring to FIG. 3, The Ericsson engine is an open cycle engine that takes in ambient air at state A and compresses it at constant temperature to state B. The air is then heated from state B to state C in a heat exchanger that allows it to recover the top cycle exhaust heat. The Ericsson expansion process state C to state D is identical to the Stirling""sxe2x80x94constant temperature with heat obtained from the top cycle exhaust.
The exhaust at state D now has no use for making mechanical work. It cannot be used in the Ericsson cycle recuperator because it would only be offset by reduced heat obtained from the top cycle exhaust (the Stirling situation). The Ericsson exhaust heat cannot be used by a spark-ignition or Diesel engine because the Ericsson exhaust is at ambient pressure and the combustion process in those topping cycles is at high pressure. If the top cycle is a gas turbine, the bottom cycle exhaust heat could be used in a recuperator heat exchanger between the top cycle compressor and combustor, but there is little sense in adding both another recuperator and a bottom cycle engine when a single recuperator could do the same job. The Ericsson cycle then has the same lost potential as the Stirling, represented by the shaded heat loss areas in FIG. 3. (The xe2x80x9clostxe2x80x9d heat could be used to heat air or water but that is not the objective here.)
FIG. 4 shows a temperature entropy diagram for the ideal bottom cycle engine. Air is taken in at ambient conditions, state A, and compressed at constant temperature to state B. The compressed air passes through a heat exchanger where the topping cycle exhaust heats it from state B to state C. The hot compressed air is then expanded, at constant entropy, to state D. With the proper compression ratio, state D and A are identical and all the available top cycle exhaust energy is converted to work.
The preceding discussion was based on ideal engines and real world engines have losses that prevent them from achieving the ideal performance. Nevertheless, it shows that the potential for a high efficiency bottom cycle engine is with an engine consisting of a cooled compressor (approximating isothermal compression), a recuperator for capturing top cycle heat, and an insulated expander (approximating constant entropy expansion).
U.S. Pat. No. 4,751,814 (xe2x80x9cAir Cycle Thermodynamic Conversion Systemxe2x80x9d, Farrell, 1988) teaches a bottom cycle engine meeting the ideal characteristics just defined. It is a turbine based system that uses multi-stage compression with intercoolers to approximate the desired constant temperature (isothermal) compression process. The compressed gas is then heated in a heat exchanger to recover the top cycle exhaust heat and expanded through a turbine to produce the work to drive the compressor and to produce useful work for an outside process. This patent also teaches the importance of establishing the flows in both the top and bottom cycles so they both have xe2x80x9cabout equal heat capacitiesxe2x80x9d. An imbalance in the heat capacities underutilizes the heat in the higher heat capacity engine and wastes otherwise available energy.
Although Farrell""s patent teaches the key thermodynamics for an ideal bottom cycle, it is a turbine based system. Such systems are viable for large powerplants but do not work well for smaller powerplants. Blade edge losses are difficult to control with smaller size turbines and the high turbine speed makes integration with the top engine shaft or electrical generators difficult. Also turbine engines cannot be built or maintained in small machine shops whereas reciprocating engines, particularly in micro-generation sizes, can easily be built and maintained in automotive machine shops.
U.S. Pat. Nos. 6,415,607; 6,301,891 and 6,216,426 (all titled xe2x80x9cHigh Efficiency, Air Bottoming Enginexe2x80x9d , Gray, Jr.; 2002, 2001 and 2001 respectively) teach substituting Farrell""s turbine cycle with reciprocating components to meet the same thermodynamic objectives. Although avoiding the problems of small turbines, Gray substitutes a very complex multiple-cylinder reciprocating arrangement. Not only is this arrangement costly to manufacture and prone to breakdown, it greatly increases the surface area of the expander and makes it much more difficult to approach the ideal, constant entropy, (adiabatic) expansion process.
U.S. Pat. No. 4,333,424 (xe2x80x9cInternal Combustion Enginexe2x80x9d, McFee, 1982) teaches a reciprocating top cycle engine consisting of a reciprocating compressor, insulated reciprocating expander, and a heat exchanger to recover the exhaust heat for preheating the compressor exit air. McFee makes use of much simpler compressor and expander geometry but is solving the problem of developing a new top cycle engine that combines the Brayton and Diesel cycles. McFee makes no mention of bottom cycle applications or how his engine could be integrated with a top cycle engine.
It is the primary aim of this invention to overcome the disadvantages of current bottom cycle engines discussed above and to achieve high rates of heat recovery from the top cycle, ease of integration with engines typical for distributed power generation, ease of control, long life, and economy of manufacture by implementing the several objects listed below.
It is an object of this invention to provide a simple, hot-air, bottom cycle engine that can be attached to any existing gas turbine, spark-ignition, Diesel or similar top cycle engine that provides a source of exhaust heat.
It is another object that the bottom cycle engine can be attached to the top cycle engine with very minor modification to the top cycle engine.
It is an additional object that the bottom cycle engine has an insignificant effect on either the efficiency or durability of the top cycle engine.
It is a still another object that the bottom cycle engine can be directly coupled mechanically to the top cycle engine so that the resulting combined cycle engine can be controlled by the existing top cycle engine controls.
It is also an object that the bottom cycle engine can be controlled independently from the top cycle engine by means of restricting the air into the bottom cycle and/or diverting the top cycle exhaust so that load following can be accomplished effectively and efficiently.
It is a further object that the bottom cycle engine can be made using already available engine blocks for most of the mechanical parts.
It is another object that it be possible to substitute some of the cylinders on a top cycle engine with bottom cycle compressor heads and/or expander heads and cylinder extenders to completely integrate the engines mechanically.
It is a further object that the bottom cycle engine can be made using commercially available compressors by mechanically connecting the compressor to the expander and/or the top cycle drive shaft(s).
It is also an object that the bottom cycle engine can be made by simple modifications to the expander cylinders of Afterburning Ericsson Cycle engines.
To implement the stated objects of the invention, a Reciprocating Hot Air Bottom Cycle Engine has been devised. The principal feature of the bottom cycle engine is its ability to approximate the ideal bottom cycle engine while being possible to construct with essentially the same methods, materials, and tools used to build conventional spark-ignition engines. The engine consists of a cooled compressor, an exhaust gas recuperator, and an insulated expander.
The compressor uses conventional air compressor technology to compress the bottom cycle air in an approximation to isothermal compression. In its simplest form, a single stage air or water-cooled reciprocating compressor can be used. Alternatively, staged compressors with inter-cooling can provide an even closer approximation to isothermal compression, although with higher manufacturing cost. Another alternative is to use rotary, Roots-blower, compressors that are simpler but less efficient. In all cases, the mechanical power to drive the compressor is obtained by mechanical connection (belt, shaft, gears etc.) to either the top cycle engine or bottom cycle expander.
The recuperator is the high effectiveness, low pressure loss, counterflow heat exchanger that recovers the exhaust heat from the top cycle and transfers it to the bottom cycle. The recuperator is identical to the recuperators used for recuperated gas turbine cycles. An excellent recuperator for this application is the Annular Flow Concentric Tube Recuperator of my U.S. Pat. No. 6,390,185.
Expanding the hot compressed gas in the expander produces the gross mechanical work of the bottom cycle engine. The expander is identical to the expander of an Afterburning Ericsson Cycle engine that has been simplified by removing the heat exchanger passages. The similarity allows both types of engines to be built in the same production line or even for simple modification from one to another in the field.
Maximum utilization of the top cycle exhaust energy is obtained by balancing the flows so that the heat capacity flowrate of both the top and bottom cycle are nearly identical. For this reason, the top and bottom engines should be matched so that they both have the same flowrates at the optimal design point. If the top and bottom cycle engines operate at the same speed, this is accomplished by simply having the total displacement of the bottom cycle compressor(s) equal to the total displacement of the top cycle engine.
The preferred means of controlling the bottom cycle engine is to mechanically couple the crankshafts of the top cycle engine and bottom cycle expander. By this means the resulting combined cycle engine can be controlled using the existing top cycle engine controls. A mechanical coupling also eliminates a means for starting the bottom cycle engine since it can be cranked by the top cycle engine, either directly by the top cycle start motor, or by engaging a clutch after the top cycle engine has started. Mechanical coupling also eliminates the need for a flywheel on the bottom cycle engine.
If it is impractical to mechanically couple the engines, the bottom cycle can be controlled by controlling the air flow and/or bypassing the top cycle exhaust flow to more closely adjust the bottom cycle output to the load demand.
A number of distinct advantages of the Reciprocating Hot Air Bottom Cycle Engine can be listed:
1. The engine provides a simple, xe2x80x9cbolt-onxe2x80x9d, means for increasing the power of existing engines with insignificant increase in fuel consumption.
2. The engine can be adapted to a wide range of top cycle engines including, but not limited to, gas turbine, spark-ignition, Diesel and even Fuel Cell powerplants.
3. Unlike a turbocharger, the bottom cycle engine can significantly increase the fuel efficiency of a spark-ignition engine because there is no need to xe2x80x9cdetunexe2x80x9d to avoid pre-ignition and detonation. This is a very important advantage to fully utilizing the potential of natural gas fuel in spark-ignition engines.
4. Unlike a turbocharger, the bottom cycle engine does not increase engine temperatures or stresses for the top cycle engine.
5. All moving parts are exposed only to clean air rather than combustion products that can limit life and performance from carbon buildup or chemical reactions.
6. The engine can be controlled independently from the top cycle engine by means of conventional throttle valves and exhaust bypass valves.
7. The sound output of the combined cycle engine is as low or lower than the original top cycle engine alone. Removing the top cycle exhaust energy through the recuperator also lowers the sound energy. The bottom cycle engine has a low exhaust pressure that does not produce an internal combustion engine""s xe2x80x9cbarkxe2x80x9d when the exhaust valve opens.